Optical pickup and optical information recording apparatus using the same

ABSTRACT

An optical pickup includes a diffraction grating partitioned into three areas, in which the phase of periodic grating groove structure in an area is successively shifted from that in the adjacent area by 90°. In the generation of a differential push-pull signal, an amplification factor K for sub push-pull signals is varied depending on the type of the optical disk. By such composition of the optical pickup, amplitude deterioration of the tracking error signal accompanying displacement of the object lens is reduced.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to an optical pickup having the functionof recording information signals on an optical information record medium(hereafter, simply referred to as “optical disk”) or reproducinginformation which has been recorded on an optical disk by irradiating aspot beam on a recording surface of the optical disk, and an opticalinformation recording/reproducing apparatus equipped with the opticalpickup.

2. Prior Art

Optical pickups are generally configured to detect a focus error signaland a tracking error signal, and control the position of an object lenswith use of the error signals so that a converged beam spot (hereafter,also referred to as a “convergence spot”) can be placed correctly on aproper recording track of an optical disk. As typical methods fordetecting the tracking error signal, a 3-spot method, a push-pullmethod, a differential push-pull method (hereafter, referred to as a“conventional DPP method” for the simplicity of explanation), etc. arewell known.

Especially, the conventional DPP method, having the advantage ofprecisely detecting the tracking error signal with a relatively simpleoptical system while realizing the detection of a reliable trackingerror signal from which offsets (caused by displacement of the objectlens and a tilt of the optical disk) have been removed satisfactorily,are widely employed mainly for optical pickups for recordable/rewritableoptical disks in recent years (see JP-A-7-272303, for example).

In the following, the principle adopted by the conventional DPP methodfor the detection of the tracking error signal will be explained brieflywith reference to FIG. 1. Incidentally, FIG. 1 will also be used laterfor the explanation of the present invention because of illustrating aconstruction element of the invention. As shown in FIG. 1, an opticalpickup employing the conventional DPP method incorporates a diffractiongrating 2 which is placed between a semiconductor laser light source 1and a half mirror 3. The diffraction grating 2, which is generallyprovided with a plurality of linear grating grooves at even pitch asshown in FIG. 2, has the function of diffracting and separating a laserbeam emitted from the semiconductor laser light source 1 into at leastthree beams containing a 0th order beam and ±1st order diffracted beams.The three beams traveling to the optical disk 10 via the half mirror 3,a collimator lens 4 and an object lens 5 are converged separately toform three convergence spots 100, 101 and 102 on the signal recordingsurface of the optical disk 10 as shown on the left side of FIG. 3. Atthis point, the positions of the three convergence spots 100, 101 and102 have been adjusted properly so that irradiation positioned intervalδ measured in the radial direction of the optical disk 10, a directionperpendicular to guide grooves 11 which are periodically formed on therecording surface of the optical disk 10, will be approximately ½ of thegroove pitch Tp between the guide grooves 11 (hereafter, the guidegroove pitch will also be called “track pitch (Tp)”) by rotating thediffraction grating 2 around its optical axis, for example. The threebeams forming the convergence spots 100, 101 and 102 are reflected bythe optical disk 10, and the reflected beams pass through the objectlens 5 and the collimator lens 4 again and reach the half mirror 3. Partof light quantity of the beams is transmitted by the half mirror 3, andthe transmitted beams are incident on a photodetector 20 through adetection lens 6.

In the photodetector 20, three photoreceptor surfaces 20 a, 20 b and 20c, each of which is divided into two or four parts, are arranged asshown on the right side of FIG. 3. The disk-reflected beams (beamsreflected by the optical disk) are separately incident on correspondingphotoreceptor surfaces and form detection beam spots 200, 201 and 202,respectively. A tracking error signal by the push-pull method(hereafter, simply referred to as “push-pull signal”) is obtained foreach detection beam spot 200, 201, 202 by letting each subtracter 50 a,50 b, 50 c execute subtraction of photoelectric signals supplied fromthe photoreceptor surfaces.

Assuming that the detection beam spots 200, 201 and 202 correspond tothe main beam spot 100, the sub spot 101 and the sub beam spot 102 onthe optical disk 10 respectively and express push-pull signals obtainedfrom the detection beam spots 200, 201 and 202 as Sa, Sb and Screspectively, the phases of the push-pull signals Sb and Sc willobviously be different from the phase of the push-pull signal Sa byapproximately 180° due to the positional relationship among theconvergence spots 100, 101 and 102 on the optical disk 10. In otherwords, the push-pull signals Sa, Sb and Sa, Sc are outputted asantiphase waveforms (push-pull signals Sb and Sc are in phase). Thus, byadding the signals Sb and Sc, and subtracting the sum Sb and Sc from thesignal Sa, an amplified tracking error signal can be obtained (notcancellation but amplification by the subtraction).

Meanwhile, the aforementioned displacement of the object lens and thetilt of the optical disk causes a certain offset component in eachpush-pull signal; however, such offset components in the push-pullsignals Sa, Sb and Sc develop obviously in the same polarity regardlessof the positions of the convergence spots 100, 101 and 102 on the disksurface. Therefore, by the aforementioned subtracting operation, theoffset components contained in the push-pull signals selectively andadvantageously cancel out, and consequently, an excellent tracking errorsignal from which the offset components have been removed perfectly orsatisfactorily can be obtained.

As shown on the right side of FIG. 3, the push-pull signals Sb and Scare added together by an adder 51, amplified properly by an amplifier52, and subtracted by an subtracter 53 from the push-pull signal Saregarding the main beam spot 100, by which the offset componentcontained in the push-pull signal Sa is removed perfectly orsignificantly and a high-quality tracking error signal with an enhancedamplitude is outputted.

The above is the signal detection principle adopted by the conventionalDPP method. Incidentally, the conventional DPP method is a well-knowntechnique which has been disclosed in JP-A-7-272303.

As explained above, the conventional DPP method has the advantage ofprecisely detecting the tracking error signal by use of a relativelysimple detecting optical system while removing the offset component ofthe tracking error signal caused by the displacement of the object lensand the tilt of the optical disk perfectly or significantly, and isespecially effective as a tracking error signal detection method foroptical pickups that are adapted to optical disks having theperiodically formed guide grooves.

However, the conventional DPP method explained above also involves thefollowing problem in practical use. In the conventional DPP method, theirradiation positioned interval δ of the three convergence spots on theoptical disk has to be adjusted to ½ of the track pitch Tp as mentionedbefore, by which the detection of satisfactory tracking error signalsbecomes difficult in cases of optical disks having track pitches Tpwidely different from 2δ.

The recordable/rewritable optical disks in rapidly increasing demandhave various types such as DVD-RAM, DVD-R and DVD-RW, in which DVD-RAMhas two types: DVD-RAM1 (track pitch Tp: about 1.48 μm, storagecapacity: about 2.6 GB) and DVD-RAM2 (track pitch Tp: about 1.23 μm,storage capacity: about 4.7 GB). Meanwhile, the track pitch is 0.74 μmin DVD-R and DVD-RW, which is exactly ½ of that of DVD-RAM1.

Recently, a versatile optical pickup, capable of reading/writing from/toall the various types of optical disks, is strongly awaited to come intopractical use. In the conventional DPP method, however, if theirradiation positioned interval δ of the three convergence spots isadjusted and optimized for the tracking error signal detection fromDVD-RAM, the irradiation positioned interval δ then become almost equalto the track pitch of DVD-R and DVD-RW, by which the tracking errorsignal detection from DVD-R and DVD-RW by the conventional DPP methodbecomes difficult. In short, if a single optical pickup is employed forvarious types of optical disks having different track pitches, thedetection of satisfactory tracking error signals by the conventional DPPmethod might become difficult or impossible for some types of disks.

In order to address the above problem, there has recently been proposeda new tracking error signal detection method capable of handling anytrack pitch and consistently detecting the tracking error signalsatisfactorily regardless of the difference in the track pitch whiletaking advantage of the merits of the conventional DPP method (e.g.JP-A-9-81942).

The tracking error signal detection method disclosed in JP-A-9-81942employs almost the same detecting optical system as that of theconventional DPP method, except for the diffraction grating 2 fordiffracting and separating the laser beam emitted by the laser lightsource into three beams. FIG. 4 shows a diffraction grating 2 employedin the document, in which the phase of periodic groove structure in afirst half area 27 is shifted from that in a second half area 28 byapproximately 180°. While description on the specific signal detectionprinciple for this method described in JP-A-9-81942 is omitted here, byputting the diffraction grating 2 having the special periodic groovepattern at the position of the diffraction grating 2 shown in FIG. 1,even if the three convergence spots 100, 101 and 102 on the optical disk10 are arranged in a single guide groove 11 as shown on the left side ofFIG. 5 differently from the arrangement in the conventional DPP method,the push-pull signal Sa obtained from the detection beam spot 200corresponding to the main beam spot 100 and the push-pull signals Sb andSc obtained from the detection beam spots 201 and 202 corresponding tothe sub spots 101 and 102 respectively, are outputted as antiphasewaveforms (Sb and Sc are in phase) as shown on the right side of FIG. 5.Therefore, a tracking error signal totally equivalent to that in theconventional DPP method can be obtained using the same arithmeticcircuitry.

The method explained above, the tracking error signal can be obtained bythe differential push-pull method, similarly to the conventional DPPmethod, even if the three convergence spots are arranged in a singleguide groove, the method enables consistent detection of an excellenttracking error signal from which the tracking offsets have been removedsatisfactorily regardless of the difference in the disk track pitch.

As explained above, the new tracking error signal detection methoddisclosed in JP-A-9-81942 (hereafter, referred to as “in-line DPPmethod” for simplifying the explanation and clarifying the object of thepresent invention) has significant advantages in that it can resolve theproblem with the conventional DPP method dependent on track pitch andrealize consistent detection of excellent tracking error signals fromvarious types of optical disks of different track pitches by a singleoptical pickup, while taking advantage of the merits of the conventionalDPP method and employing similar arithmetic circuitry.

However, the in-line DPP method still involves a major problem inpractical use as described below.

In the in-line DPP method, if the object lens is displaced or shifted ina direction substantially perpendicular to the recording tracks of theoptical disk (hereafter, simply referred to as “tracking direction”),the amplitude of obtained tracking error signals drops significantly asthe displacement increases. The deterioration of properties of thetracking error signal accompanying the object lens displacement in thetracking direction (hereafter called “object lensdisplacement-to-tracking error signal ratio characteristic”) is by farmore remarkable in the in-line DPP method than in the conventional DPPmethod. Thus, in order to obtain a usable tracking error signal by theconventional in-line DPP method, the object lens displacement has to belimited to an extremely narrow permissible range, by which practicalperformance of the optical pickup is necessitated to be impairedseriously at present.

The above problem regarding the “object lens displacement-to-trackingerror signal ratio characteristic” has not be mentioned in publiclyknown documents at all and, as a matter of course, no countermeasure hasbeen disclosed.

SUMMARY OF THE INVENTION

It is therefore the object of the present invention to provide a newtracking error signal detection unit capable of resolving the mainproblem with the in-line DPP method having deterioration of propertiesof the tracking error signal accompanying the object lens displacement,while maintaining the aforementioned advantages of the in-line DPPmethod, that is, capable of consistently detecting a practical trackingerror signal from which tracking offsets has been satisfactorily removedfrom various types of optical disks having different track pitches,being less affected by the difference in the disk track pitch and theobject lens displacement in the tracking direction. The presentinvention especially aims to realize a tracking error signal detectionunit with improved versatility and reliability, and to provide anoptical pickup and an optical information recording/reproducingapparatus employing such a tracking error signal detection unit.

In accordance with an aspect of the present invention, there is providedan optical pickup comprising: a laser light source; a beam separationunit which separates a laser beam emitted by the laser light source intoat least three beams; a converging optical system which converges thethree beams and thereby forms three separate convergence spots on arecording surface of an optical disk; and a photodetector which isplaced to receive each of reflected beams of the three convergence spotsfrom the optical disk with a photoreceptor surface divided into at leasttwo faces. In the optical pickup, the beam separation unit is dividedinto at least three areas (first through third areas, with the firstarea placed between the second and third areas) each of which hasprescribed periodic structure. The second area is formed to haveperiodic structure that is shifted from that of the first area byapproximately 90° in the phase of the periodic structure, and the thirdarea is formed to have periodic structure that is shifted from that ofthe second area by approximately 180° in the phase of the periodicstructure.

Preferably, the three convergence spots are formed so that theirradiation positioned interval between adjacent convergence spotsmeasured in a direction substantially orthogonal to guide groovesperiodically formed on the recording surface of the optical informationrecord medium will be approximately equal to zero or an integralmultiple of the interval between the guide grooves.

In accordance with another aspect of the present invention, an opticalinformation recording/reproducing apparatus at least incorporates theaforementioned optical pickup and a tracking error signal detection unithaving the function of detecting a tracking error signal according to adifferential push-pull method by executing proper operations to signalsobtained from the photoreceptor surfaces of the photodetector of theoptical pickup.

In accordance with another aspect of the present invention, an opticalinformation recording/reproducing apparatus comprises: an optical pickupincorporating a laser light source, a beam separation unit whichseparates a laser beam emitted by the laser light source into a mainbeam and at least two sub beams, a converging optical system whichconverges the main beam and the sub beams and thereby forms threeseparate convergence spots on a recording surface of an opticalinformation record medium on which guide grooves have been formed atpreset intervals, and a photodetector which is placed to receive each ofreflected beams of the three convergence spots from the opticalinformation record medium with a photoreceptor surface divided into atleast two faces; a push-pull signal generation circuit which generatespush-pull signals regarding the main beam and the sub beams respectivelyby executing proper operations to photoelectric signals obtained fromthe photoreceptor surfaces of the optical pickup; a differentialpush-pull signal generation circuit which generates a differentialpush-pull signal by adding all or part of the push-pull signalsregarding the sub beams together, amplifying the added signal by anamplification factor K, and subtracting the amplified signal from thepush-pull signal regarding the main beam; and an amplification factorcontrol unit which changes the amplification factor K depending on thepitch between the guide grooves of the optical information recordmedium.

BRIEF DESCRIPTION OF THE DRAWINGS

The objects and features of the present invention will become moreapparent from the consideration of the following detailed descriptiontaken in conjunction with the accompanying drawings.

FIG. 1 is a schematic block diagram showing an optical pickup inaccordance with an embodiment of the present invention;

FIG. 2 is a perspective view showing an example of a diffraction gratingemployed in the conventional DPP method;

FIG. 3 is a schematic diagram showing the arrangement of convergencespots on the optical disk and the outline composition of the detectingsystem employed in the conventional DPP method;

FIG. 4 is a perspective view showing an example of a diffraction gratingemployed in the conventional in-line DPP method;

FIG. 5 is a schematic diagram showing the arrangement of convergencespots on the optical disk and the outline composition of the detectingsystem employed in the conventional in-line DPP method;

FIG. 6 is a perspective view showing an example of a diffraction gratingemployed in the present invention;

FIG. 7 is a schematic diagram showing an example of the arrangement ofconvergence spots on the optical disk and the outline composition of thedetecting system employed in the present invention;

FIG. 8 is a perspective view showing wavefront shapes of a transmittedbeam and diffracted beams when the diffraction grating of the presentinvention is used;

FIG. 9 is a schematic diagram showing examples of wavefront shapes andintensity distribution of beam spots on the detector surface in theconventional in-line DPP method;

FIG. 10 is a schematic diagram showing sectional forms of wavefronts of±1st order diffracted beams when displacement of the object lensoccurred in the conventional in-line DPP method;

FIG. 11 is a schematic diagram showing examples of wavefront shapes andintensity distribution of beam spots on the detector surface when acertain amount of object lens displacement occurred in the conventionalin-line DPP method;

FIG. 12 is a schematic diagram showing sectional forms of wavefronts of±1st order diffracted beams when displacement of the object lensoccurred in the present invention;

FIG. 13 is a schematic diagram showing examples of wavefront shapes andintensity distribution of beam spots on the detector surface when acertain amount of object lens displacement occurred in the presentinvention;

FIG. 14 is a graph showing the “object lens displacement-to-trackingerror signal ratio characteristic” when a DVD-RAM1 disk is played backby optical pickups according to the present invention and theconventional in-line DPP method;

FIG. 15 is a graph showing the “object lens displacement-to-trackingerror signal ratio characteristic” when a DVD-RAM2 disk is played backby optical pickups according to the present invention and theconventional in-line DPP method;

FIG. 16 is a graph showing the relationship between the object lensdisplacement and residual offtrack error after the servo pull-in when aDVD-RAM1 disk is played back by the present invention;

FIG. 17 is a graph showing the relationship between the object lensdisplacement and residual offtrack error after the servo pull-in when aDVD-RAM2 disk is played back by the present invention;

FIGS. 18A and 18B are layout plans showing another embodiment of thepresent invention concerning the arrangement of convergence spots;

FIG. 19 is a schematic block diagram showing an optical pickup inaccordance with another embodiment of the present invention;

FIG. 20 is a schematic block diagram showing an optical pickup inaccordance with another embodiment of the present invention; and

FIG. 21 is a block diagram showing an example of an optical informationrecording/reproducing apparatus employing the optical pickup accordingto the present invention.

DETAILED DESCRIPTION OF THE EMBODIMENTS

Referring now to the drawings, a description will be given in detail ofembodiments in accordance with the present invention. While FIG. 1,schematically showing elements of the present invention, has alreadybeen used for the description of the prior art, it will be used againfor the explanation of the following embodiments. In the followingdescription, elements which have already been explained using referencenumerals will be referred to the same reference numerals.

FIG. 1 is a schematic block diagram showing an example of an opticalpickup in accordance with the present invention, in which the referencenumeral 1 denotes a semiconductor laser light source, 3 denotes a halfmirror or beam splitter, 4 denotes a collimator lens, 5 denotes anobject lens, 6 denotes a detection lens, 10 denotes an optical disk, and20 denotes a photodetector having photoreceptor surfaces partitionedaccording to a prescribed pattern. The object lens 5, being fixed in alens holder 15, is driven in the direction of the optical axis (focusdirection) and in the tracking direction by a two-dimensional actuator25 composed of electromagnetic circuits.

Between the semiconductor laser 1 and the half mirror 3, a diffractiongrating 2 in accordance with the present invention is placed. Adiffraction grating 2 (details are shown in FIG. 6) is different fromthat of FIG. 4 which has been explained in the description of the priorart. The details of the diffraction grating 2 will be explained later. Alaser beam emitted by the semiconductor laser 1 is diffracted andseparated by the diffraction grating 2 into at least three beamsincluding 0th order beam and ±1st order diffracted beams (unshown). Theseparated beams are reflected by the half mirror 3, reach the objectlens 5 via the collimator lens 4, and are separately converged by theobject lens 5 on the recording surface of the optical disk 10 to formthree convergence spots 100, 101 and 102.

The three convergence spots 100, 101 and 102 formed on the recordingsurface of the optical disk 10 at this point are arranged substantiallyin a line so that they will be in one of the guide grooves 11periodically formed on the optical disk 10, as shown on the left side ofFIG. 7.

The three beams forming the convergence spots 100, 101 and 102 arereflected by the optical disk 10 and travel reversely on almost the sameoptical path to the half mirror 3 through the object lens 5 and thecollimator lens 4. Part of light quantity of the beams is transmitted bythe half mirror 3, and the transmitted beams are incident onphotoreceptor surfaces of the multi-face photodetector 20 via thedetection lens 6. Signals detected by the photoreceptor surfaces of thephotodetector 20 are processed by proper arithmetic circuitry andthereby object lens position control signals such as focus error signal,tracking error signal, etc., and information signals which have beenstored on the recording surface of the optical disk 10 are obtained. Thetracking error signal is obtained from signals detected by photoreceptorsurfaces 20 a, 20 b and 20 c of the photodetector 20 by use of exactlythe same arithmetic circuitry such as subtracters 50 a, 50 b and 50 c,adder 51, amplifier 52, subtracter 53, etc. as those of the conventionalDPP method, as shown on the right side of FIG. 7.

In each example shown in FIGS. 3, 5 or 7, the photodetector has aplurality of photoreceptor surfaces, each of which is separated into atleast two faces in regard to a direction corresponding to the so-calledtangential direction of the disk perpendicular to the radial directionof the disk. The push-pull signal regarding each convergence spot isobtained from signals outputted by the two-face photoreceptor surface,by subtracting one from the other. Generally, a push-pull signal isobtained from the difference between two output signals of a two-facephotoreceptor surface that is separated into two faces in regard to adirection corresponding to the radial direction of the disk. However,since the examples shown in FIGS. 3, 5 and 7 all employ an astigmaticmethod for detecting the focus error signal, each beam spot on eachphotoreceptor surface of the photodetector has intensity distributionthat has been rotated around the optical axis by approximately 90°,therefore, the push-pull signal in the examples of FIGS. 3, 5 and 7 isobtained from the difference between two output signals of the two-facephotoreceptor surface that is separated into two faces in regard to thedirection corresponding to the tangential direction of the disk.Incidentally, the features and advantages of such photoreceptor surfacearrangement of the photodetector, in the case where a focus error signaldetection unit employing the astigmatic method is combined with atracking error signal detection unit employing the push-pull method ordifferential push-pull method as above, have already been publiclyknown.

While the first embodiment of the present invention shown in FIGS. 1 and7 has almost the same composition of the optical system and convergencespot arrangement as those in the conventional in-line DPP method whichhas been explained referring to FIG. 5 etc., the grating pattern of thediffraction grating 2 placed between the semiconductor laser 1 and thehalf mirror 3 is especially different from that of the in-line DPPmethod.

FIG. 6 is a perspective view showing a grating pattern of thediffraction grating 2 employed in the present invention. While aplurality of grooves are formed at even intervals as usual on thegrating surface of the diffraction grating 2, the grating surface ispartitioned into at least three areas by parting lines that areorthogonal to the grooves as shown in FIG. 6. In other words, thegrating surface is partitioned into at least three areas 31, 32 and 33in a direction corresponding to the tracking direction of the opticaldisk 10. The central area 32 is given a prescribed width W. The phase ofthe periodically formed grooves of the area 31 adjoining the centralarea 32 is differentiated from that of the central area 32 by +90°, thatis, the groove arrangement in the area 31 is shifted from that in thecentral area 32 by approximately ¼ of the groove interval. Meanwhile,the phase of the periodically formed grooves of the area 33 adjoiningthe central area 32 on the opposite side is differentiated from that ofthe central area 32 by −90°, that is, the groove arrangement in the area33 is reversely shifted from that in the central area 32 byapproximately ¼ of the groove interval. Thus, the groove arrangement inthe area 31 is shifted from that in the area 33 by a phase difference of180° or half a pitch.

Each 1st order diffracted beam is diffracted and separated by thespecial grating pattern of the diffraction grating 2 to receive acertain type of modulation to its wavefront (equiphase surface). FIG. 8is a schematic diagram showing the wavefront modulation given by thediffraction grating 2. For example, when a beam 130 with a planewavefront 140 is incident on the diffraction grating 2, each of thewavefronts (equiphase surfaces) 161 and 162 of the +1st and −1st orderdiffracted beams 151 and 152 diffracted and separated by the diffractiongrating 2, has a step-like shape having three stages with leveldifferences of λ/4 (corresponding to phase differences of 90°) as shownin FIG. 8. In this case, the width of the central area of the 3-stagewavefront equals the width W of the central area 32 of the diffractiongrating 2. As shown in FIG. 8, the shapes of the wavefronts 161 and 162have reversed convexity/concavity and phase variation. On the otherhand, the grating pattern has no effect on the 0th order beam 150 whichpasses through the grating intact, and thus the 0th order beam 150 has aplane wavefront (equiphase surface) 160 like the incident beam 130.Incidentally, while the above explanation has been given assuming thatthe beam 130 incident on the diffraction grating 2 is a parallel beamwith a plane wavefront (equiphase surface) for the sake of simplicity,even if a diverging beam emitted by the semiconductor laser light source1 is directly incident on the diffraction grating 2, that is, even whenthe beam incident on the diffraction grating 2 has a more or lessspherical wavefront as in the embodiment of FIG. 1, the diffractiongrating 2 gives the ±1st order diffracted beams similar phase modulationand the 3-stage wavefront shapes.

In the following, an explanation will be given on the reason why the“object lens displacement-to-tracking error signal ratio characteristic”is more improved by the above embodiment employing the specialdiffraction grating partitioned into three areas and using the ±1storder diffracted beams having the 3-stage wavefronts (equiphasesurfaces) rather than by the conventional in-line DPP method even thoughthe same tracking error signal detection unit is used.

First, the principle adopted by the in-line DPP method for the trackingerror signal detection will be explained briefly.

As mentioned before, the conventional in-line DPP method employs thediffraction grating 2 having two areas (see FIG. 4) for diffracting andseparating the laser beam emitted by the semiconductor laser lightsource 1 into three beams. By diffracting and separating the laser beamusing such a diffraction grating 2, each ±1st order diffracted beam isgiven a step-like wavefront having two stages with a level difference ofλ/2 corresponding to a phase difference of 180°, travels through thecollimator lens 4 and the object lens 5 maintaining the state, and isconverged on the recording surface of the optical disk (Hereafter, theconvergence spots of the ±1st order diffracted beams on the optical diskwill be simply referred to as “sub beam spots”.).

Meanwhile, the 0th order beam passes through the diffraction grating 2intact receiving no phase modulation to its wavefront, is collimated bythe collimator lens 4 into a parallel beam having a plane wavefront,enters the object lens 5 as the parallel beam, and is converged on therecording surface of the optical disk similarly to the ±1st orderdiffracted beams (Hereafter, the convergence spot of the 0th order beamon the optical disk will be simply referred to as “main beam spot”.).

Incidentally, when a laser beam is converged on the recording surface ofthe optical disk having the periodic guide grooves, the laser beam beingreflected by the disk is diffracted by the guide grooves and isseparated into at least three beams: a 0th order beam and ±1st orderdiffracted beams. The 0th order beam and the ±1st order diffracted beamsfrom the disk travel through the pupil plane of the object lensoverlapping with one another and having certain deviations from oneanother, and reach a photoreceptor surface of the photodetector. An areaon the photoreceptor surface where the 0th order beam and a ±1st orderdiffracted beam overlap with each other, has a certain level of lightintensity due to the interference between the beams. In this case,whether the light intensity is high or low (light or dark) is decided byrelative phase difference between the wavefronts of the overlapping 0thorder beam and ±1 st order diffracted beam, and the phase differencesamong the 0th order beam wavefront and the ±1st order diffracted beamwavefronts change successively depending on the position of theconvergence spot relative to the guide grooves of the disk.

FIG. 9 is a schematic diagram showing the phase relationship among thewavefronts of the 0th order beam and ±1st order diffracted beams,diffracted and separated by the guide grooves of the disk, which areoverlapping with one another in the detection beam spot on eachphotoreceptor surface while having certain deviations from one anotherwhen the disk-reflected beams from the main beam spot and the sub beamspots reach the photoreceptor surfaces of the photodetector in theconventional in-line DPP method employing the special diffractiongrating having two areas, and statuses of light intensity (light ordark) caused by interference of beams depending on the phaserelationship, classifying the cases by the positional relationshipbetween the convergence spot and the guide grooves on the disk.

Regarding the main beam spot shown in FIG. 9, in the detection beam spot200 on the photoreceptor surface, the ±1st order diffracted beamsoverlap with the 0th order beam having certain rightward/leftwarddeviations from the 0th order beam as mentioned above. The wavefronts ofthe 0th order beam and the ±1st order diffracted beams all havesubstantially plane shapes. In states (A) and (C) of FIG. 9 where theconvergence spot is located at the center of a guide groove 11 or at themidpoint between two guide grooves 11, the wavefronts of the ±1st orderdiffracted beams caused by the guide grooves have phase differences of+90° or −90° relative to the wavefront of the 0th order beam. As theconvergence spot shifts gradually from the position or state, the phaserelationship among the wavefronts also change successively. In a state(B) where the convergence spot has shifted by ¼ of the groove pitch andin a state (D) where the convergence spot has reversely shifted by ¼ ofthe groove pitch, one of the ±1st order diffracted beam wavefrontscaused by the guide grooves has a phase difference of 0° relative to the0th order beam wavefront and the other has a phase difference of 180°relative to the 0th order beam wavefront. Further, the states (B) and(D) have inverse phase difference relationships between right and left(the overlap area of the +1st order diffracted beam and the 0th orderbeam and the overlap area of the −1st order diffracted beam and the 0thorder beam).

Thus, the light intensity in the overlap area of the +1st orderdiffracted beam and the 0th order beam and that in the overlap area ofthe −1st order diffracted beam and the 0th order beam vary continuouslydepending on the positional relationship between the convergence spotand the guide grooves on the disk, and further, the variations occur inthe right and left overlap areas perfectly inversely. Therefore, thetracking error signal according to the so-called push-pull method can beobtained by letting the photodetector and the photoreceptor surfacepartitioned into at least two separately detect the light intensityvariations in the right and left overlap areas and output a differentialsignal.

Regarding the sub beam spots in the conventional in-line DPP method, thewavefront of each beam has a step-like shape having two stages with alevel difference of λ/2 corresponding to a phase difference of 180° asmentioned before, and the step-like wavefront shape remains unchangedeven if the beam is reflected by the optical disk till the beam reachesthe photoreceptor surface of the photodetector. Similarly to the case ofthe main beam spot, the ±1st order diffracted beams diffracted andseparated by the guide grooves of the disk overlap with the 0th orderbeam having certain rightward/leftward deviations from the 0th orderbeam, and the light intensity in the overlapping part changes due tointerference depending on the relative phase difference between the 0thorder beam wavefront and the +1st order diffracted beam wavefront andthat between the 0th order beam wavefront and the −1st order diffractedbeam wavefront. It is also totally similar to the case of the main beamspot that average phase difference between the 0th order beam wavefrontand each 1st order diffracted beam wavefront caused by the guide groovesexhibits certain variation depending on the positional relationshipbetween the convergence spot and the guide grooves on the disk (states(A) to (D)).

However, in the case of the sub beam spots, since each wavefront has astep-like shape having two stages with a level difference of λ/2corresponding to a phase difference of 180° as mentioned before, even ifthe average phase difference between the 0th order beam wavefront andeach ±1st order diffracted beam wavefront determined by the positionalrelationship between the convergence spot and the guide grooves on thedisk (states (A) to (D)) exhibited exactly the same behavior as in thecase of the main beam spot, resultant changes of the light intensity inthe right/left interference areas become totally opposite to those inthe case of the main beam spot, as shown in FIG. 9. This means that inthe in-line DPP method, the phase of waveform of the push-pull signalobtained from each sub beam spot becomes totally opposite to that of thepush-pull signal obtained from the main beam spot even if the sub beamspots and the main beam spot had exactly the same positions relative tothe guide grooves on the disk. The above mechanism enables the in-lineDPP method, although employing the in-line convergence spot arrangementwith the main beam spot and the sub beam spots located in the same guidegroove, to obtain the tracking error signals like those in theconventional DPP method.

Next, in a case where the object lens is displaced or shifted in thetracking direction on radial direction of the optical disk by a certainshift amount S in the above-described in-line DPP method, therenaturally occurs a shift corresponding to the shift amount S between thecentral optical axis of the object lens 5 and a boundary line betweenthe areas 27 and 28 of the diffraction grating 2, as shown in FIG. 10.As a result, there also occurs a shift between the central optical axisof the object lens 5 and a boundary line, where the level differenceexists, on the wavefront 161 of the ±1st order diffracted beam emergingfrom the diffraction grating 2, causing a difference of width betweenthe two wavefront areas having the λ/2 level difference corresponding to180° phase difference. Further, when the +1st or −1st order diffractedbeam is reflected by the disk, the wavefront 161 of the beam isconverted into a wavefront 171 of a disk-reflected beam as shown in FIG.10. Since the wavefront 171 of the disk-reflected beam has a shape thatis obtained by asymmetrically reversing the wavefront 161 of thedisk-incident beam with respect to the central optical axis of theobject lens as the symmetry axis, the wavefront 171 of the disk-incidentbeam entering the photoreceptor surface also has the difference of widthbetween its two areas having the λ/2 level difference corresponding to180° phase difference, similarly to the wavefront 161 of thedisk-incident beam.

FIG. 11 is a schematic diagram showing the phase relationship among thewavefronts of the 0th order beam and ±1st order diffracted beams,diffracted and separated by the guide grooves of the disk, which areoverlapping with one another in the detection beam spot 201 (or 202) onthe photoreceptor surface 20 b (or 20 c) while having certain deviationsfrom one another when a sub beam spot generated by converging a 1storder diffracted beam, whose wavefront has come to have the leveldifference between its two wavefront areas and asymmetry with respect tothe central optical axis of the object lens due to the displacement ofthe object lens, has formed the detection beam spot 201 (or 202) on thephotoreceptor surface 20 b (or 20 c) of the photodetector after beingreflected by the disk, and statuses of light intensity caused byinterference of beams depending on the phase relationship, classifyingthe cases by the positional relationship between the convergence spotand the guide grooves on the disk.

The following points become clear by comparing FIGS. 11 and 9. Referringfirst to FIG. 9 with no displacement of the object lens, in the states(B) and (D) where the convergence spot has shifted from a guide grooveby ¼ of the groove interval, the phase difference between the wavefrontsof the 0th order beam and each 1st order diffracted beam after beingdiffracted and separated by the guide grooves is fixed to 0° or 180°throughout each right/left overlap area, by which the light intensitybecomes full-light or full-dark throughout each overlap area.

Meanwhile, referring to FIG. 11 with the width difference between thetwo wavefront areas having the λ/2 level difference corresponding to180° phase difference due to displacement of the object lens, even inthe states (B) and (D), the phase difference between the wavefronts ofthe 0th order beam and each 1st order diffracted beam changes from 0° to180° or from 180° to 0° even in the right/left overlap areas. As aresult, a dark part appears in each light overlap area and a light partappears in each dark overlap area. By the appearance of theintensity-inverted parts in the overlap areas, the push-pull signalobtained from a difference signal between the signals detected by theright and left photoreceptor faces obviously loses its modulationdegree. The above is the primary cause of the deterioration of the“object lens displacement-to-tracking error signal ratio characteristic”occurring more significantly in the conventional in-line DPP methodemploying the diffraction grating having two areas rather than in theconventional DPP method.

In contrast to the conventional in-line DPP method which has beenexplained above, the present invention employs the aforementioneddiffraction grating 2 partitioned into three parts. By use of such adiffraction grating 2, a +1st order diffracted beam from the diffractiongrating 2 incident on the optical disk and a +1st order diffracted beamafter being reflected by the optical disk both have a step-likewavefront having three stages with level differences of λ/4corresponding to phase differences of 90° as shown in FIGS. 8 and 12.Incidentally, while the wavefront of each (disk-incident/disk-reflected)−1st order diffracted beam is not shown in FIG. 12, the −1st orderdiffracted beam wavefront has a shape (convexity/concavity) reverse tothe +1st order diffracted beam wavefront as is clear from FIG. 8.

FIG. 13 is a schematic diagram showing the phase relationship among thewavefronts of the 0th order beam and ±1st order diffracted beams,diffracted and separated by the guide grooves of the disk, which areoverlapping with one another in the detection beam spot 201 on thephotoreceptor surface 20 b while having certain deviations from oneanother when the 3-stage wavefront has been formed by use of the 3-areadiffraction grating 2 of the present invention and the +1st orderdiffracted beam, having the wavefront which has become asymmetrical withrespect to the central optical axis of the object lens 5 due to thedisplacement of the object lens 5 in the disk radial direction as shownin FIG. 12, has been converged by the object lens 5 on the recordingsurface of the optical disk as a sub beam spot and formed the detectionbeam spot 201 on the photoreceptor surface 20 b of the photodetector 20after being reflected by the optical disk, and statuses of lightintensity caused by interference of beams depending on the phaserelationship, classifying the cases by the positional relationshipbetween the convergence spot and the guide grooves on the disk.

As is clear from the comparison between FIGS. 13 and 11, thedistribution of light intensity in the right/left interference areas inFIG. 13 with the 3-area diffraction grating 2 of the present inventionis obviously different from that in FIG. 11 with the conventional 2-areadiffraction grating. For example, in the parts which have been called“intensity-inverted parts” in the states (B) and (D) of FIG. 11, thephase difference between the 0th order beam and ±1st order diffractedbeam diffracted and separated by the guide grooves becomes ±90° in thepresent invention and the parts have intermediate light intensitybetween that of the light area and that of the dark area. While theabove explanation was actually only about the +1st order diffractedbeam, out of the two ±1st order diffracted beams diffracted andseparated by the diffraction grating 2 of the present invention, as amatter of course, a similar phenomenon occurs regarding the −1st orderdiffracted beam. As a result, in each push-pull signal obtained fromeach sub beam spot by the present invention, the modulation degreedeterioration rate relative to the object lens displacement is reducedmuch compared to the conventional in-line DPP method employing the2-area diffraction grating, by which the “object lensdisplacement-to-tracking error signal ratio characteristic” is improvedsignificantly.

FIG. 14 is a graph showing the relationship between the object lensdisplacement in the tracking direction and the amplitude of the trackingerror signal detected by the in-line DPP method (hereafter, the trackingerror signal will be simply referred to as “DPP signal”), that is, the“object lens displacement-to-tracking error signal ratio characteristic”of the DPP signal, which has been obtained by computer simulation for acase where a DVD-RAM1 disk (storage capacity: 2.6 GB, guide grooveinterval: 1.48 μm) is played back by an optical pickup having parameterswhich will be mentioned below. In FIG. 14, the horizontal axis denotesthe displacement of the object lens 5 and the vertical axis denotesrelative amplitude of the DPP signal, in which relative amplitude 100%equals the DPP signal amplitude when the conventional in-line DPP methodwith the 2-area diffraction grating is employed and the object lensdisplacement is 0. In addition to the result for the conventionalin-line DPP method, FIG. 14 also shows the “object lensdisplacement-to-tracking error signal ratio characteristics” for threecases where the in-line DPP method of the present invention, with the3-area diffraction grating, is employed. In the three cases, the width Wof the central area 32 of the 3-area diffraction grating 2 measured inthe direction corresponding to the disk radial direction was changed, inwhich the width W, converted into the width of the corresponding centralarea of the wavefront of the beam just before entering the object lens5, was set to 0.3 mm, 0.5 mm and 0.8 mm.

Key parameters of the optical pickup used for the computer simulationwere as below:

-   -   (1) laser beam wavelength: 660 nm    -   (2) power: about 6.3×    -   (3) object lens NA: about 0.64

As seen in FIG. 14, in the three cases employing the in-line DPP methodof the present invention, with the 3-area diffraction grating, the DPPsignal amplitude near the 0 displacement line, object lensdisplacement=0, is lower than in the conventional in-line DPP methodwith the 2-area diffraction grating; however, the deterioration rate ofDPP signal amplitude with respect to the object lens displacement isreduced, which means the “object lens displacement-to-tracking errorsignal ratio characteristic” is improved by the in-line DPP method ofthe present invention. Incidentally, the drop in the DPP signalamplitude near the 0 displacement line is insignificant since it can becompensated for by amplifying the outputted DPP signal by an amplifierwith a proper amplification factor.

Next, FIG. 15 is a graph showing the “object lensdisplacement-to-tracking error signal ratio characteristic” of the DPPsignal amplitude detected by the in-line DPP method, which has beenobtained by computer simulation for a case where a DVD-RAM2 disk(storage capacity: 4.7 GB, guide groove interval: 1.23 μm) is playedback by an optical pickup having the same parameters as those in FIG.14. In FIG. 15, the horizontal and vertical axes and parameters areexactly the same as those of FIG. 14. As is clear from FIG. 15, thein-line DPP method according to the present invention, with the 3-areadiffraction grating, satisfactorily improves the “object lensdisplacement-to-tracking error signal ratio characteristic” also in theplayback of DVD-RAM2, in contrast with the conventional in-line DPPmethod.

As seen in FIGS. 14 and 15, in the in-line DPP method of the presentinvention employing the 3-area diffraction grating, both in DVD-RAM1playback and DVD-RAM2 playback, the deterioration rate of the DPP signalamplitude with respect to the object lens displacement is more reducedas the width of the central area 32 of the 3-area diffraction grating 2gets larger i.e. as W gets larger in FIGS. 14 and 15. However, adetailed examination revealed that increasing the central area width Wof the 3-area diffraction grating 2 too much results in severerdistortion in the DPP signal waveform. By extensive numericalcalculations and examinations, a suitable range of the central areawidth W, converted into the width of the corresponding central area ofthe wavefront of the beam just before entering the object lens 5, wasfound to be 10% to 40% (preferably, 20% to 30%) of the aperture of theobject lens 5.

While results for the playback of DVD-R/RW are not particularly shown,an excellent “object lens displacement-to-tracking error signal ratiocharacteristic” as in the conventional DPP method can be obtained by thein-line DPP method of the present invention employing the 3-areadiffraction grating.

Incidentally, the above computer simulation of FIGS. 14 and 15 wasperformed assuming, for simplifying calculations, that the sum of lightquantities detected at the two sub beam spots, convergence spots formedon the optical disk by the ±1st order diffracted beams diffracted andseparated by the diffraction grating 2, is equal to a light quantitydetected at the main beam spot, convergence spot formed on the opticaldisk by the 0th order beam transmitted by the diffraction grating 2intact, and the DPP signal was calculated as:

$\begin{matrix}{{{DPP}\mspace{14mu}{signal}} = \left\lbrack {{push}\text{-}{pull}\mspace{14mu}{signal}\mspace{14mu}{obtained}\mspace{14mu}{from}\mspace{14mu}{main}}\mspace{14mu} \right.} \\{\left. {{beam}\mspace{14mu}{spot}} \right\rbrack - \left\lbrack {{sum}\mspace{14mu}{of}\mspace{14mu}{push}\text{-}{pull}\mspace{14mu}{signals}} \right.} \\\left. {{obtained}\mspace{14mu}{from}\mspace{14mu}{two}\mspace{14mu}{sub}\mspace{14mu}{beam}\mspace{14mu}{spots}} \right\rbrack\end{matrix}$However, in actual optical pickups, there obviously occurs a differenceof light quantity between the main beam spot and the sub beam spots dueto a difference of diffraction efficiency of the diffraction grating 2between the 0th order beam and the ±1st order diffracted beams, etc. Asa result, a great difference of amplitude arises in the push-pullsignals obtained from the convergence spots. For this reason, when theDPP signal is obtained from an actual optical pickup, a properamplification factor K is generally introduced to the push-pull signalsobtained from the sub beam spots and the DPP signal is calculated as:

$\begin{matrix}{{{DPP}\mspace{14mu}{signal}} = \left\lbrack {{push}\text{-}{pull}\mspace{14mu}{signal}\mspace{14mu}{obtained}\mspace{14mu}{from}\mspace{14mu}{main}}\mspace{14mu} \right.} \\{\left. {{beam}\mspace{14mu}{spot}} \right\rbrack - {K \times \left\lbrack {{sum}\mspace{14mu}{of}\mspace{14mu}{push}\text{-}{pull}\mspace{14mu}{signals}} \right.}} \\\left. {{obtained}\mspace{14mu}{from}\mspace{14mu}{two}\mspace{14mu}{sub}\mspace{14mu}{beam}\mspace{14mu}{spots}} \right\rbrack\end{matrix}$The amplification factor K can be analyzed into:K=K1×K2where K1 can be expressed as:

$\begin{matrix}{{K1} = {\left\lbrack {{light}\mspace{14mu}{quantity}\mspace{14mu}{detected}\mspace{14mu}{at}\mspace{14mu}{main}\mspace{14mu}{beam}\mspace{14mu}{spot}} \right\rbrack/}} \\{\left\lbrack {{sum}\mspace{14mu}{of}\mspace{14mu}{light}\mspace{14mu}{quantities}\mspace{14mu}{detected}\mspace{14mu}{at}\mspace{14mu}{two}\mspace{14mu}{sub}\mspace{14mu}{beam}\mspace{14mu}{spots}} \right\rbrack}\end{matrix}$

Meanwhile, K2 is a compensation coefficient for compensating for thedifference between the modulation degree of the push-pull signalobtained from the main beam spot and the modulation degree of thepush-pull signal obtained from the sub beam spots.

In the conventional DPP method and the conventional in-line DPP methodemploying the 2-area diffraction grating, the modulation degree of thepush-pull signal obtained from the sub beam spots has generally beenequated with the modulation degree of the push-pull signal obtained fromthe main beam spot, that is, the compensation coefficient K2 has beenneglected or set to K2=1, K=K1.

However, in the in-line DPP method of the present invention employingthe 3-area diffraction grating 2, there occurs an obvious differencebetween the modulation degree of the push-pull signal obtained from themain beam spot and the modulation degree of the push-pull signalobtained from the sub beam spots (generally, the former is larger thanthe latter). Thus, it is desirable that K2 be set to a proper valuelarger than 1. Further, we found out that the optimum value of K2changes depending on the track pitch (guide groove pitch) of the disk.

FIG. 16 is a graph showing the relationship between the object lensdisplacement and residual offtrack error of the DPP signal, deviation ofthe tracking servo pull-in point or zero-cross point of the DPP signaldetected by the optical pickup from the true center of the guide groove,in the playback of a DVD-RAM1 disk, calculated for several values of K2.The optical pickup parameters used for the calculation are the same asthose in the computer simulation of FIGS. 14 and 15.

As shown in FIG. 16, while the residual offtrack error developsconsiderably as the object lens displacement increases in theconventional setting K2=1.0, the residual offtrack error decreasesproportionally to the increasing K2. However, detailed calculations madeit clear that too large K2 causes severer distortion in the DPP signalwaveform, similarly to the case where the central area width W of the3-area diffraction grating 2 is increased too much. From ourexaminations, the optimum value of K2 for the playback of DVD-RAM1 disksis approximately 1.4 to 1.6.

FIG. 17 is a graph similarly showing the relationship between the objectlens displacement and the residual offtrack error of the DPP signalcalculated for several values of K2, in the playback of a DVD-RAM2 disk.Also in the playback of DVD-RAM2, the residual offtrack erroraccompanying the object lens displacement varies according to the changein K2. However, in the case of DVD-RAM2, the optimum value of K2 isapproximately 1.2 to 1.4, and by setting K2 within the range, theresidual offtrack error can be almost perfectly eliminated in the objectlens displacement range between −0.3 mm and +0.3 mm.

While results for the playback of DVD-R/RW (guide groove interval: 0.74μm) are not particularly shown, the conventional setting K2=1.0 asoptimum value eliminates the residual offtrack error almost perfectly inthe object lens displacement range between −0.3 mm and +0.3 mm.

Incidentally, also in the conventional in-line DPP method shown in FIG.4 employing the 2-area diffraction grating, it is possible to reduce theresidual offtrack error satisfactorily by varying the optimum value ofK2 depending on the type of the disk.

The above is the outline of the principle of the present invention. Inthe following, another embodiment according to the present inventionwill be described. While the main beam spot and the two sub beam spotswere simultaneously formed in the same guide groove of the optical diskin the above embodiment shown in FIGS. 1 and 7, the beam spotarrangement is not limited to the in-line arrangement shown in FIG. 7.The DPP signal can be obtained by exactly the same unit as that in thein-line DPP method of the present invention as long as the distancebetween the main beam spot and the sub beam spot measured in the diskradial direction is set to an integral multiple of the track pitch ofthe disk. For example, assuming that the distance between the main beamspot 100 and each sub beam spot 101, 102 in the disk radial direction isset approximately equal to one guide groove pitch 1.48 μm of DVD-RAM1 asshown in FIG. 18A for the playback of a DVD-RAM1 disk, if a DVD-R/RWdisk is played back with the same beam spot arrangement, the main beamspot 100 and sub beam spots 101 and 102 are simultaneously formedprecisely at the centers of separate guide grooves as shown in FIG. 18B,by which the tracking error signal detection by the in-line DPP methodalso becomes possible. Incidentally, when a DVD-RAM2 disk of the guidegroove pitch: 1.23 μm is played back with the same beam spot arrangementplacing the main beam spot at the center of a guide groove, each subbeam spot slightly deviates from the center of an adjacent guide groove.However, the deviation is almost negligible and no major problem arisesin the detection of the tracking error signal by the in-line DPP method.Further, as a matter of course, the distance between the main beam spot100 and each sub beam spot 101, 102 in the disk radial direction is notlimited to the above value. For example, the distance may also be set toapproximately 1.36 μm, which is in between 1.48 μm of the guide groovepitch of DVD-RAM1 and 1.23 μm of the guide groove pitch of DVD-RAM2.

In the following, still another embodiment according to the presentinvention will be described referring to FIG. 19. FIG. 19 is a schematicblock diagram showing another example of an optical pickup in accordancewith the present invention, in which the same reference numerals asthose of FIG. 1 in the first embodiment designate the same components asthose of FIG. 1.

While the diffraction grating 2 of the present invention was placedbetween the semiconductor laser light source 1 and the half mirror 3 inthe first embodiment shown in FIG. 1, the position of the diffractiongrating 2 is not particularly limited as long as it is on the opticalpath between the light source and the object lens. In the example ofFIG. 19, the diffraction grating 2 is placed right in front of theobject lens 5 and fixed in the lens holder 15 together with the objectlens 5, allowing the object lens 5 to be driven by the two-dimensionalactuator 25 together with the object lens 5. By such composition of theoptical pickup, the aforementioned deterioration of the “object lensdisplacement-to-tracking error signal ratio characteristic” accompanyingthe object lens displacement can be reduced further since the positionalrelationship between the object lens 5 and the diffraction grating 2does not change even if the object lens displacement occurred.

Incidentally, in the above composition of FIG. 19 with the diffractiongrating 2 placed right in front of the object lens 5, not only goingbeams, the disk-incident beams traveling from the semiconductor laserlight source 1 to the optical disk 10, but also returning beams, thedisk-reflected beams traveling from the optical disk 10 to thephotodetector 20, pass through the diffraction grating 2. Therefore, ifa diffraction grating having ordinary diffraction characteristics isused, the returning beams are further diffracted by the diffractiongrating 2, causing unnecessary stray light and deteriorating performanceof the pickup. The problem can be resolved by employing a diffractiongrating 2 having anisotropic diffraction characteristics, for example, adiffraction grating diffracting a beam having a particular polarizationdirection with certain diffraction efficiency while perfectlytransmitting a beam having a polarization direction orthogonal to theparticular polarization direction without generating diffracted light,and placing a quarter wave plate 40 on the optical path between thediffraction grating 2 and the object lens 5 for orthogonalizing thepolarization directions of the going and returning beams.

In addition, while the optical elements of the optical pickup werearranged separately in the above embodiments, the present invention isnot limited to such optical pickups. In an example shown in FIG. 20 (inwhich the same reference numerals as those of FIGS. 1 and 19 are usedfor optical elements equivalent to those of the previous embodiments),the so-called “semiconductor laser module” is employed, in which thesemiconductor laser light source 1, photodetector 20, etc. are assembledinto one package 41.

In the example of FIG. 20, the package 41 is sealed up by putting atransparent substrate 42 with a proper thickness at the opening of thepackage 41 where the laser beam emitted by the light source 1 emerges.On the upper surface of the transparent substrate 42 is a hologramelement 43, having the functions of separating the optical paths of thegoing beams traveling from the light source 1 to the optical disk 10 andthe returning beams traveling from the optical disk 10 to thephotodetector 20 and leading the returning beams to the photodetector20. The diffraction grating 2 of the present invention is placed at thelower surface of the transparent substrate 42.

The optical pickup employing the above semiconductor laser module, inwhich the semiconductor laser light source 1, the photodetector 20, etc.are assembled into one package 41, has the advantage of reducing thesize and thickness of the optical pickup.

Incidentally, while concrete shapes and arrangement of the photoreceptorsurfaces of the photodetector 20 are not particularly shown in FIG. 20,the semiconductor laser module can of course have a variety ofcomposition as long as it can detect the tracking error signal by thedifferential push-pull method.

It goes without saying that the application of the present invention isnot restricted to the optical pickups which have been shown in FIGS. 1,19 and 20. The present invention is applicable to various opticalpickups of a wide range of composition as long as they employ trackingerror signal detection units according to the differential push-pullmethod.

Lastly, an embodiment of an optical information recording/reproducingapparatus employing the optical pickup of the present invention is shownin FIG. 21. The optical information recording/reproducing apparatus ofFIG. 21 can also be implemented as a separate recording apparatus or aseparate reproducing apparatus.

In FIG. 21, the reference numeral 60 denotes an optical pickup havingthe composition of FIGS. 1, 19 or 20, for example. The optical pickup 60is provided with a mechanism for sliding in the radial direction of theoptical disk 10, and the position of the optical pickup 60 is controlledby an access control signal supplied from an access control circuit 72.

A semiconductor laser light source in the optical pickup 60 is suppliedwith a drive current from a laser drive circuit 76 and thereby emits alaser beam with a proper light quantity. Various servo signals andinformation signals detected by photodetectors of the optical pickup 60are sent to a servo signal generation circuit 74 and an informationsignal reproduction circuit 75. The servo signal generation circuit 74generates the focus error signal and the tracking error signal from thedetected signals. An actuator drive circuit 73 drives a two-dimensionalactuator in the optical pickup 60 based on the focus error signal andthe tracking error signal, by which position control of the object lens5 is carried out. The information signal reproduction circuit 75reproduces the information signal which has been recorded in the opticaldisk 10, by use of detected signals.

Part of the signals obtained by the servo signal generation circuit 74and the information signal reproduction circuit 75 are supplied to acontrol circuit 70. The control circuit 70 is connected to the laserdrive circuit 76, the access control circuit 72, a spindle motor drivecircuit 71, etc., by which, the control of emission light quantity ofthe semiconductor laser of the optical pickup 60, the control of accessdirection/position, the control of a spindle motor 77 for revolving theoptical disk 10, etc. are realized. The control circuit 70 includes adisk discrimination circuit (unshown), which recognizes the type of theoptical disk using signals obtained by the servo signal generationcircuit 74 and the information signal reproduction circuit 75. Based onthe disk type recognition, a gain (corresponding to K2) for subpush-pull signals (push-pull signals obtained from the sub beam spots)to be adopted by a DPP signal generation circuit (unshown) of the servosignal generation circuit 74 is controlled automatically, for example.

As set forth hereinabove, by the present invention, an excellent andpractical tracking error signal, in which the amplitude deterioration oftracking error signal and the residual offtrack error accompanying thedisplacement of the object lens have been reduced satisfactorily, can beobtained in the recording/playback of various types of optical diskshaving different track pitches. Therefore, an optical pickup with highversatility and high reliability and an optical informationrecording/reproducing apparatus employing such an optical pickup can berealized.

It should be further understood by those skilled in the art thatalthough the foregoing description has been made on embodiments of theinvention, the invention is not limited thereto and various changes andmodifications may be made without departing from the spirit of theinvention and the scope of the appended claims.

1. An optical pickup comprising: a laser light source; a beam separationmeans which separates a laser beam emitted by the laser light sourceinto at least three beams; a converging optical system which convergesthe three beams and forms three separate convergence spots on arecording surface of an optical information record medium; and aphotodetector which is placed to receive each of reflected beams of thethree convergence spots from the optical information record medium witha photoreceptor surface divided into at least two faces, wherein: thebeam separation means is divided into at least three areas, firstthrough third areas, each of which has prescribed periodic structure,and the first area is placed between the second and third areas, and thesecond area has periodic structure that is shifted from that of thefirst area by approximately 90° in the phase of the periodic structure,and the third area has periodic structure that is shifted from that ofthe second area by approximately 180° in the phase of the periodicstructure.
 2. The optical pickup according to claim 1, wherein the threeconvergence spots are formed such that the interval between adjacentconvergence spots measured in a direction substantially orthogonal toguide grooves periodically formed on the recording surface of theoptical information record medium is approximately equal to zero or anintegral multiple of a pitch between the guide grooves.
 3. An opticalinformation recording/reproducing apparatus for reading or writinginformation from/to an optical information record medium by laser beamirradiation, comprising: an optical pickup including a laser lightsource, a beam separation means which separates a laser beam emitted bythe laser light source into at least three beams, a converging opticalsystem which converges the three beams and forms three separateconvergence spots on a recording surface of the optical informationrecord medium, and a photodetector which is placed to receive each ofreflected beams of the three convergence spots from the opticalinformation record medium with a photoreceptor surface divided into atleast two faces; and a tracking error signal detection means having thefunction of detecting a tracking error signal according to adifferential push-pull method by executing proper operations to signalsobtained from the photoreceptor surfaces of the photodetector of theoptical pickup, wherein: the beam separation means is partitioned intoat least three areas, first through third areas, each of which hasprescribed periodic structure, and the first area is placed between thesecond and third areas, and the second area has periodic structure thatis shifted from that of the first area by approximately 90° in the phaseof the periodic structure, and the third area has periodic structurethat is shifted from that of the second area by approximately 180° inthe phase of the periodic structure.